Generate cstubs for a "vendored" library.

We have a dummy C library hosted entirely in the 'vendor' directory and use
the ctypes instrumentation and description language to generate bindings for
it.

This is the version that builds into an executable.

This test is identical to exe-vendored.t except it overrides the
generated_types stanza field.

  $ LIBEX=$(realpath "$PWD/../libexample")
  $ TARGET=./vendor
  $ mkdir -p $TARGET && install $LIBEX/* $TARGET
  $ dune exec ./example.exe
  4
